The present invention relates to a method and apparatus for the compression of digital video signals and in particular to the use of such method and apparatus in the encoding and decoding of signals.
Variable bit rate (VBR) video compression is known to give advantages over constant bit-rate (CBR) video compression. The main reason for this is that in a CBR system the bit rate has to be set so that the worst case quality is acceptable while in a VBR system the bit rate is set so that the average quality (which is kept constant) is acceptable. The difference between the average bit rate of a VBR compressed signal compared to the bit rate of a CBR compressed signal has been found to be to be close to 30% in favour of VBR compression.
Though VBR compression is better than CBR, it can only be used in a limited number of applications. In principle the medium should be able to convey variable bit-rate signals. In terms of the ISO/OSI model, almost every medium conveys a fixed bit rate at the physical layer. On a higher level, the medium can be converted into a (logically) variable bit rate medium.
One particular application of VBR compression is joint bit-rate control, in which a number of sources make use of a single channel. In the case of joint bit-rate control the bit-rate of the video signals is controlled such that the individual bit rates can be varying (through optimal allocation of bit rate) but so that the sum of all bit rates is constant. This type of system may occur in for instance cable television or in satellite television services.
The idea of joint bit-rate control for multi-program video signal encoding has been found to be advantageous, particularly for video signals coded according to the ISO MPEG standards. Basically a system for joint bit-rate control needs technical measures for two problems, namely the bit allocation and the buffer management. To deal with bit allocation, the bit need of the programs is measured and the bits are spread accordingly over the programs. Buffer management encompasses several sub-problems, some of which are described in “Constraints on variable bit-rate video for ATM-networks” by Amy Reibman and Barry Haskell; in IEEE Transactions on Circuits and Systems for Video Technology, Vol 2, No. 4 December 1992 pp. 361-372. The Reibman and Haskell paper examines the constraints resulting from encoder and decoder buffering in an asynchronous transfer mode (ATM) network, in particular the additional constraints needed to prevent overflowing or underflowing of the decoder buffer when a variable bit-rate data channel links encoder and decoder. They describe a method in which the number of encoded bits for each video frame and the number of bits transmitted across the variable bit-rate channel are selected jointly, a necessity imposed by the differing constraints imposed on the transmitted bit rate by the encoder and decoder buffers respectively.
In all these applications the transmission of video can be in the MPEG format. An MPEG decoder contains a physical buffer, and correct MPEG bit streams must fulfil the video buffering verifier (VBV) constraints, which means that the signals may not overflow or underflow a hypothetical decoder buffer. As will be shown hereinafter, an incorrect buffer management may limit the performance of a VBR compression system. The analyses of buffering systems rely strongly on the concept of a system delay whereby, in order to have a continuous display of video, the system delay must be constant. This requirement has an effect on the buffering strategy.
For CBR systems, given a fixed decoder buffer size, the delay must be relatively large for a low bit-rate and relatively low for a high bit-rate in order to make full use of the available buffering space in the decoder. If the bit rate is variable a compromise setting of the buffering system must be used. As a consequence of this compromise the effective buffer size in the decoder is too low for an acceptable performance at low bit rates. Essentially, two things are required, namely a constant end-to-end delay, and a low buffering delay for high bit rates and a high buffering delay for low bit rates. These requirements would appear to be incompatible.
It is an object of the present invention to provide improved stability of decoder buffer operation.
It is a further object of the present invention to provide greater efficiency in encoder buffer management.